Covalent Bonding of Carboxylated Cellulose Fiber Webs

ABSTRACT

Methods are provided for creating covalent bonding of webs by combining cellulosic fibers having a carboxyl content approximately greater than 7 meq/100 g with one or more crosslinking agents. In a first step, a carboxyl group is placed onto a fiber. In an embodiment, the fiber is then reacted with an oxazoline-functional polymer which has been combined with a polycarboxylate compound. Heat is applied to the treated web, and this enables formation of a cross-linked bridge in the form of a covalent bond. In an embodiment, the covalent bonding of the carboxylated cellulose pulp webs utilizes oxazoline-functional polymers and polyacrylic acid. The oxazoline polymer in combination with polyacrylic acid should form a network polymer with covalent bonds to the cellulose carboxyl groups. The non-woven web may be strengthhened by covalent bonding, thereby improving overall wet/dry strength of the final product.

FIELD OF THE INVENTION

The present invention generally relates to methods for providingcovalent bonds on cellulose fiber webs.

BACKGROUND OF THE INVENTION

Cellulose fibers are generally held together by hydrogen bonds. Theaverage energy of a hydrogen bond is 1-5 Kcal. The strength of a paperproduct is typically related to the strength of the hydrogen bonding.Often times when attempts are made to strengthen the bonding of fibers,other properties are compromised, such as bulk, stiffness, etc. In somecases, increasing bond strength can increase the overall cost of theproduct, which is undesirable.

Thus, a need exists for a method for increasing bond strength betweencellulose fibers without compromising properties of the final product.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention are described in detail belowwith reference to the following drawings.

FIG. 1 is a diagram of a system for forming covalent bonds in anembodiment of the present invention; and

FIG. 2 is a representation of Epocros polymers in an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for creating covalent bonding ofwebs by combining cellulosic fibers having a carboxyl contentapproximately greater than 7 meq/100 g with one or more crosslinkingagents. In a first step, a carboxyl group is placed onto a fiber. In anembodiment, the fiber is then reacted with an oxazoline-functionalpolymer which has been combined with a polycarboxylate compound. Heat isapplied to the treated web, and this enables formation of a cross-linkedbridge in the form of a covalent bond. In an embodiment, the covalentbonding of the carboxylated cellulose pulp webs utilizesoxazoline-functional polymers and polyacrylic acid. The oxazolinepolymer in combination with polyacrylic acid should form a networkpolymer with covalent bonds to the cellulose carboxyl groups. Thenon-woven web may be strengthened by covalent bonding, thereby improvingoverall wet/dry strength of the final product.

FIG. 2 illustrates a general class of polymers that have beenfunctionalized with an oxazoline group.

Conventional papermaking fiber may be utilized and a furnish for thesame may refer to papermaking fibers made from any species, includinghardwoods and softwoods, and to fibers that may have had a debonderapplied to them but that are not otherwise chemically treated followingthe pulping/bleaching process or off-line post pulping/bleaching &drying process. The cellulose fiber may be obtained from any source,including cotton, hemp, grasses, cane, husks, cornstalks or othersuitable source. In an embodiment, the cellulose fiber is chemical woodpulp.

The oxazoline-functional polymers may be, for example, any polymercontaining an oxazoline containing moiety on the side chain. In place ofoxazoline containing polymers, one can use a polyfunctional compoundcapable of reacting to carboxyl groups (e.g. polyols, polyepoxides,etc.).

The polycarboxylate compound may be, for example, a polymer or oligomercontaining multiple carboxyl groups.

The crosslinking agent can include a catalyst to accelerate the bondingreaction between the crosslinking agent and the cellulose molecule, butmost crosslinking agents do not require a catalyst. Suitable catalystsinclude acidic salts which can be useful when urea-based crosslinkingsubstances are used. Such salts include ammonium chloride, ammoniumsulfate, aluminum chloride, magnesium chloride, or mixtures of these orother similar compounds. Alkali metal salts of phosphorus containingacids may also be used.

The crosslinking agent typically is applied in an amount ranging fromabout 8 kg to about 100 kg chemical per ton of cellulose fiber. Thepolycarboxylate compound is applied in an amount ranging from about 8 kgto about 100 kg chemical per ton of cellulose fiber.

The cellulosic fibers may have been treated with a debonding agent priorto treatment with the crosslinking agent. Debonding agents tend tominimize interfiber bonds and allow the fibers to separated from eachother more easily. The debonding agent may be cationic, non-ionic oranionic. Cationic debonding agents appear to be superior to non-ionic oranionic debonding agents. The debonding agent typically is added tocellulose fiber stock.

Suitable cationic debonding agents include quaternary ammonium salts.These salts typically have one or two lower alkyl substituents and oneor two substituents that are or contain fatty, relatively long chainhydrocarbon. Non-ionic debonding agents typically comprise reactionproducts of fatty-aliphatic alcohols, fatty-alkyl phenols andfatty-aromatic and aliphatic acids that are reacted with ethylene oxide,propylene oxide or mixtures of these two materials.

Examples of debonding agents may be found in Hervey et al U.S. Pat. Nos.3,395,708 and 3,544,862, Emanuelsson et al U.S. Pat. No. 4,144,122,Forssblad et al U.S. Pat. No. 3,677,886, Osborne III U.S. Pat. No.4,351,699, Hellston et al U.S. Pat. No. 4,476,323 and Laursen U.S. Pat.No. 4,303,471 all of which are in their entirety incorporated herein byreference. A suitable debonding agent is Berocell 584 from BerolChemicals, Incorporated of Metairie, La. It may be used at a level of0.25% weight of debonder to weight of fiber. Again, a debonding agentmay not be required.

In FIG. 1, a conveyor 12 transports a cellulosic mat 14 into a treatmentzone 16 where an applicator 18 applies a crosslinking agent onto the mat14. Typically, chemicals are applied optionally to both sides of themat. The mat 14 is then conveyed into a dryer 20 followed by a flowthrough oven 22 to cure the crosslinking agent.

The treated pads have low density and good stiffness. The pads can becut easily using a sharp knife. The material is absorbent and strongeven when wet.

The present invention may be better understood by way of the followingexamples. It should be understood that, in the following examples, toproduce the desired carboxyl groups in meq/100 g for experimentation,processes described in U.S. Pat. Nos. 6,379,494; 6,352,348 and 6,919,447were utilized.

EXAMPLE 1 Ratios on Epocros WS500 and Polyacrylic Acid

Fluff pulp modified to have a carboxyl content of 21 meq/100 g was usedto make a 6 inch airlaid pad at 125 gsm. The carboxylated pulp can be ineither a neutralized form or in a fully protonated (acid) form. The padswere sprayed with 10 gm of a solution of oxazoline functionalizedpolyacrylate (Epocros WS500) manufactured by Nippon Shokubai andpolyacrylic acid (MW˜3500) from Rohm&Haas, to yield the required levelof Epocros and polyacrylic acid shown in the table below. The Epocroslevel was varied from 3% to 7% based on fiber weight and the polyacrylicacid from 1% to 5% based on fiber weight. The control pads containedonly polyacrylic acid. The pads were dried and cured in a convectionoven at 120° C. for 10 minutes. The pads were then tested for wet anddry tensile strength using an Instron testing device/system with avertical pull. For the wet tensile, the pads were sprayed with 10 gm ofdeionized water, let stand for 10 minutes, then tested.

From Table 1 it can be seen that there is a substantial increase in thedry tensile index with higher strength values for increasing polymercontent. The data also show that higher strength values are obtainedfrom the acid form of the carboxylated pulp. Similar results are shownin Table 2 for the wet tensile index.

TABLE 1 Dry tensile Index, Nm/g Percent Epocros Percent Polyacrylic AcidWS500 1% 3% 5% Tensile Index for acidic form of pulp Control 0.83 1.202.02 3% 1.17 3.62 2.38 5% 1.12 5.70 3.56 7% 1.53 5.08 4.18 Tensile Indexfor neutralized form of pulp Control 0.83 1.20 2.02 3% 2.65 3.62 2.29 5%4.10 3.20 4.40 7% 3.28 3.74 5.15

TABLE 2 Wet Tensile Index, Nm/g Percent Epocros Percent Polyacrylic AcidWS500 1% 3% 5% Tensile Index for acidic form of pulp Control 0.17 0.350.49 3% 0.33 1.01 0.65 5% 0.32 1.21 1.18 7% 0.41 1.82 1.24 Tensile Indexfor neutralized form of pulp Control 0.17 0.35 0.49 3% 0.39 0.50 0.37 5%0.89 0.71 0.69 7% 0.54 0.67 0.75

EXAMPLE 2 Effect of Increasing Carboxyl Content in Pulp

Fluff pulp modified to have a carboxyl content from 3 to 35 meq/100 gwas used to make a 6 inch airlaid pad at 125 gsm. The carboxylated pulpcan be in either a neutralized form or in a fully protonated (acid)form. The pads were sprayed with 10 gm of a solution of Epocros WS500and polyacrylic acid (MW˜3500) from Rohm&Haas, to yield the requiredlevel of Epocros and polyacrylic acid(PAA) shown in the table below. TheEpocros level was varied from 3% to 7% based on fiber weight and thepolyacrylic acid was held at 3% based on fiber weight. The pads weredried and cured in a convection oven at 120° C. for 10 minutes. The padswere then tested for wet and dry tensile strength using an Instrontesting device/system with a vertical pull. For the wet tensile, thepads were sprayed with 10 gm of deionized water, let stand for 10minutes, then tested.

From Table 3 it can be seen that there is a substantial increase in thedry tensile index with an increase in the Epocros content, and there isan optimum carboxyl level. It is also apparent that the acid form of thecarboxylated pulp is more reactive, yielding higher tensile strengths.Similar results are shown in Table 4 for the wet tensile index.

TABLE 3 Dry Tensile Index, Nm/g Percent Epocros - PAA Carboxyl contentof pulp, meq/100 g on pulp 3 12 21 35 Tensile Index for acid form 3-31.13 1.59 3.62 1.62 5.3 1.45 1.60 5.70 2.15 7.3 2.14 1.82 5.08 4.01Tensile Index for neutralized form 3-3 1.22 1.75 2.93 2.83 5-3 1.45 2.213.20 2.87 7-3 2.16 2.97 3.74 3.97

TABLE 4 Wet Tensile Index, Nm/g Percent Epocros - PAA Carboxyl contentof pulp, meq/100 g on pulp 3 12 21 35 Tensile Index for acid form 3-30.25 0.35 1.01 0.48 5.3 0.30 0.45 1.21 0.54 7.3 0.39 0.49 1.82 0.81Tensile Index for neutralized form 3-3 0.25 0.34 0.50 0.48 5-3 0.29 0.480.71 0.54 7-3 0.38 0.65 0.67 0.87

EXAMPLE 3 Ratios on Epocros WS500 and Polymaleic Acid

Fluff pulp modified to have a carboxyl content of 21 meq/100 g was usedto make a 6 inch airlaid pad at 125 gsm. The carboxylated pulp can be ineither a neutralized form in a fully protonated (acid) form. The padswere sprayed with 10 gm of a solution of Epocros WS500 and polymaleicacid (MW˜3500) from Rohm&Haas, to yield the required level of Epocrosand polymaleic acid shown in the table below. The Epocros level wasvaried from 3% to 7% based on fiber weight and the polymaleic acid from1% to 5% based on fiber weight. The control pads contained onlypolymaleic acid. The pads were dried and cured in a convection oven at120° C. for 10 minutes. The pads were then tested for wet and drytensile strength using an Instron testing device/system with a verticalpull. For the wet tensile, the pads were sprayed with 10 gm of deionizedwater, let stand for 10 minutes, then tested.

From Table 5 it can be seen that there is a substantial increase in thedry tensile index with higher strength values for increasing polymercontent. The data also show that higher strength values are obtainedfrom the acid form of the carboxylated pulp. Similar results are shownin Table 6 for the wet tensile index.

TABLE 5 Dry tensile Index, Nm/g Percent Epocros Percent Polyacrylic AcidWS500 1% 3% 5% Tensile Index for acidic form of pulp Control 0.53 1.201.35 3% 0.66 3.65 1.95 5% 1.56 3.9 2.64 7% 2.37 3.67 3.04 Tensile Indexfor neutralized form of pulp Control 0.53 1.20 1.35 3% 2.507 1.64 1.845% 2.04 2.26 1.75 7% 2.50 2.66 3.19

TABLE 6 Wet Tensile Index, Nm/g Percent Epocros Percent Polyacrylic AcidWS500 1% 3% 5% Tensile Index for acidic form of pulp Control 0.15 0.320.40 3% 0.21 0.69 0.49 5% 0.47 1.23 0.74 7% 0.86 1.24 1.13 Tensile Indexfor neutralized form of pulp Control 0.15 0.32 0.40 3% 0.32 0.31 0.34 5%0.48 0.47 0.45 7% 0.62 0.61 0.72

The Epocros is described as an oxazoline functionalized polymer. Theparticular polymer backbone used in the example here is a polyacrylateco-polymer. Other heating methods beyond those listed above arecontemplated which will accelerate the reaction. These methods are knownby those skilled in the art. The temperature range for heating may beapproximately 60 degrees Celsius to 150 degrees Celsius. Curing for theprocess may occur via heat and/or pressure.

While the embodiments of the invention have been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is not limited by the disclosure of the embodiments.Instead, the invention should be determined entirely by reference to theclaims that follow.

1. A method of covalent bonding of a cellulosic web, the methodcomprising the steps of: forming a web from highly carboxylatedcellulose fibers; applying a cross-linking agent to the highlycarboxylated cellulose fibers to create a treated web; and curing thetreated web.
 2. The method of claim 1 wherein the treated web is curedunder heat.
 3. The method of claim 1 wherein the treated web is curedunder pressure.
 4. The method of claim 1 wherein a carboxyl content ofthe fibers is approximately greater than 7 meq/100 grams.
 5. The methodof claim 1 wherein the crosslinking agent is a mixture of oxazolinefunctionalized polymer and polycarboxylate compound.
 6. The method ofclaim 1 wherein the crosslinking agent is applied in an amount rangingfrom about 8 kg to about 100 kg chemical per ton of highly carboxylatedcellulose fibers.
 7. The method of claim 5 wherein the polycarboxylatecompound is applied in an amount ranging from about 8 kg to about 100 kgchemical per ton of highly carboxylated cellulose fibers.
 8. Acellulosic web comprising: a highly carboxylated cellulose fiber; afunctionalized polymer; and a polycarboxylate material; wherein thefunctionalized polymer and the polycarboxylate material form across-linking agent and further wherein a covalent bond is formedbetween the highly carboxylated cellulose fiber and the cross-linkingagent.
 9. The cellulosic web of claim 8 wherein the functionalizedpolymer is an oxazoline-containing polymer.
 10. The cellulosic web ofclaim 8 wherein the functionalized polymer is applied in an amountranging from about 8 kg to about 100 kg chemical per ton of highlycarboxylated cellulose fibers.
 11. The cellulosic web of claim 8 whereinthe polycarboxylate material is applied in an amount ranging from about8 kg to about 100 kg chemical per ton of highly carboxylated cellulosefibers.
 12. The cellulosic web of claim 8 wherein a carboxyl content ofthe highly carboxylated cellulose fibers is approximately greater than 7meq/100 grams.